\begin{tikzpicture}
    \tkzDefPoints{0/1/A,  4/1/B}
    \tkzDefPoints{0/0/C,  3/0/D}

    \tkzDrawSegments[xianduan={below=0pt}](A,B)
    \tkzDrawSegments[xianduan={below=0pt}](C,D)
    \tkzLabelSegment[above](A,B){$a$}
    \tkzLabelSegment[above](C,D){$b$}
\end{tikzpicture}
